DESCRIPTION

Given
node
and
service,
which identify an Internet host and a service,
getaddrinfo()
returns one or more
addrinfo
structures, each of which contains an Internet address
that can be specified in a call to
bind(2)
or
connect(2).
The
getaddrinfo()
function combines the functionality provided by the
gethostbyname(3)
and
getservbyname(3)
functions into a single interface, but unlike the latter functions,
getaddrinfo()
is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.

The
addrinfo
structure used by
getaddrinfo()
contains the following fields:

The
hints
argument points to an
addrinfo
structure that specifies criteria for selecting the socket address
structures returned in the list pointed to by
res.
If
hints
is not NULL it points to an
addrinfo
structure whose
ai_family,
ai_socktype,
and
ai_protocol
specify criteria that limit the set of socket addresses returned by
getaddrinfo(),
as follows:

ai_family

This field specifies the desired address family for the returned addresses.
Valid values for this field include
AF_INET
and
AF_INET6.
The value
AF_UNSPEC
indicates that
getaddrinfo()
should return socket addresses for any address family
(either IPv4 or IPv6, for example) that can be used with
node
and
service.

ai_socktype

This field specifies the preferred socket type, for example
SOCK_STREAM
or
SOCK_DGRAM.
Specifying 0 in this field indicates that socket addresses of any type
can be returned by
getaddrinfo().

ai_protocol

This field specifies the protocol for the returned socket addresses.
Specifying 0 in this field indicates that socket addresses with
any protocol can be returned by
getaddrinfo().

ai_flags

This field specifies additional options, described below.
Multiple flags are specified by bitwise OR-ing them together.

All the other fields in the structure pointed to by
hints
must contain either 0 or a null pointer, as appropriate.

Specifying
hints
as NULL is equivalent to setting
ai_socktype
and
ai_protocol
to 0;
ai_family
to
AF_UNSPEC;
and
ai_flags
to
(AI_V4MAPPED | AI_ADDRCONFIG).
(POSIX specifies different defaults for
ai_flags;
see NOTES.)
node
specifies either a numerical network address
(for IPv4, numbers-and-dots notation as supported by
inet_aton(3);
for IPv6, hexadecimal string format as supported by
inet_pton(3)),
or a network hostname, whose network addresses are looked up and resolved.
If
hints.ai_flags
contains the
AI_NUMERICHOST
flag, then
node
must be a numerical network address.
The
AI_NUMERICHOST
flag suppresses any potentially lengthy network host address lookups.

If the
AI_PASSIVE
flag is specified in
hints.ai_flags,
and
node
is NULL,
then the returned socket addresses will be suitable for
bind(2)ing
a socket that will
accept(2)
connections.
The returned socket address will contain the "wildcard address"
(INADDR_ANY
for IPv4 addresses,
IN6ADDR_ANY_INIT
for IPv6 address).
The wildcard address is used by applications (typically servers)
that intend to accept connections on any of the hosts's network addresses.
If
node
is not NULL, then the
AI_PASSIVE
flag is ignored.

If the
AI_PASSIVE
flag is not set in
hints.ai_flags,
then the returned socket addresses will be suitable for use with
connect(2),
sendto(2),
or
sendmsg(2).
If
node
is NULL,
then the network address will be set to the loopback interface address
(INADDR_LOOPBACK
for IPv4 addresses,
IN6ADDR_LOOPBACK_INIT
for IPv6 address);
this is used by applications that intend to communicate
with peers running on the same host.

service
sets the port in each returned address structure.
If this argument is a service name (see
services(5)),
it is translated to the corresponding port number.
This argument can also be specified as a decimal number,
which is simply converted to binary.
If
service
is NULL, then the port number of the returned socket addresses
will be left uninitialized.
If
AI_NUMERICSERV
is specified in
hints.ai_flags
and
service
is not NULL, then
service
must point to a string containing a numeric port number.
This flag is used to inhibit the invocation of a name resolution service
in cases where it is known not to be required.

Either
node
or
service,
but not both, may be NULL.

The
getaddrinfo()
function allocates and initializes a linked list of
addrinfo
structures, one for each network address that matches
node
and
service,
subject to any restrictions imposed by
hints,
and returns a pointer to the start of the list in
res.
The items in the linked list are linked by the
ai_next
field.

There are several reasons why
the linked list may have more than one
addrinfo
structure, including: the network host is multihomed, accessible
over multiple protocols (e.g., both
AF_INET
and
AF_INET6);
or the same service is available from multiple socket types (one
SOCK_STREAM
address and another
SOCK_DGRAM
address, for example).
Normally, the application should try
using the addresses in the order in which they are returned.
The sorting function used within
getaddrinfo()
is defined in RFC 3484; the order can be tweaked for a particular
system by editing
/etc/gai.conf
(available since glibc 2.5).

If
hints.ai_flags
includes the
AI_CANONNAME
flag, then the
ai_canonname
field of the first of the
addrinfo
structures in the returned list is set to point to the
official name of the host.

The remaining fields of each returned
addrinfo
structure are initialized as follows:

*

The
ai_family,
ai_socktype,
and
ai_protocol
fields return the socket creation parameters (i.e., these fields have
the same meaning as the corresponding arguments of
socket(2)).
For example,
ai_family
might return
AF_INET
or
AF_INET6;
ai_socktype
might return
SOCK_DGRAM
or
SOCK_STREAM;
and
ai_protocol
returns the protocol for the socket.

*

A pointer to the socket address is placed in the
ai_addr
field, and the length of the socket address, in bytes,
is placed in the
ai_addrlen
field.

If
hints.ai_flags
includes the
AI_ADDRCONFIG
flag, then IPv4 addresses are returned in the list pointed to by
res
only if the local system has at least one
IPv4 address configured, and IPv6 addresses are returned
only if the local system has at least one IPv6 address configured.
The loopback address is not considered for this case as valid
as a configured address.
This flag is useful on, for example,
IPv4-only systems, to ensure that
getaddrinfo()
does not return IPv6 socket addresses that would always fail in
connect(2)
or
bind(2).

If
hints.ai_flags
specifies the
AI_V4MAPPED
flag, and
hints.ai_family
was specified as
AF_INET6,
and no matching IPv6 addresses could be found,
then return IPv4-mapped IPv6 addresses in the list pointed to by
res.
If both
AI_V4MAPPED
and
AI_ALL
are specified in
hints.ai_flags,
then return both IPv6 and IPv4-mapped IPv6 addresses
in the list pointed to by
res.
AI_ALL
is ignored if
AI_V4MAPPED
is not also specified.

The
freeaddrinfo()
function frees the memory that was allocated
for the dynamically allocated linked list
res.

Extensions to getaddrinfo() for Internationalized Domain Names

Starting with glibc 2.3.4,
getaddrinfo()
has been extended to selectively allow the incoming and outgoing
hostnames to be transparently converted to and from the
Internationalized Domain Name (IDN) format (see RFC 3490,
Internationalizing Domain Names in Applications (IDNA)).
Four new flags are defined:

AI_IDN

If this flag is specified, then the node name given in
node
is converted to IDN format if necessary.
The source encoding is that of the current locale.

If the input name contains non-ASCII characters, then the IDN encoding
is used.
Those parts of the node name (delimited by dots) that contain
non-ASCII characters are encoded using ASCII Compatible Encoding (ACE)
before being passed to the name resolution functions.

AI_CANONIDN

After a successful name lookup, and if the
AI_CANONNAME
flag was specified,
getaddrinfo()
will return the canonical name of the
node corresponding to the
addrinfo
structure value passed back.
The return value is an exact copy of the value returned by the name
resolution function.

If the name is encoded using ACE, then it will contain the
xn--
prefix for one or more components of the name.
To convert these components into a readable form the
AI_CANONIDN
flag can be passed in addition to
AI_CANONNAME.
The resulting string is encoded using the current locale's encoding.

AI_IDN_ALLOW_UNASSIGNED, AI_IDN_USE_STD3_ASCII_RULES

Setting these flags will enable the
IDNA_ALLOW_UNASSIGNED (allow unassigned Unicode code points) and
IDNA_USE_STD3_ASCII_RULES (check output to make sure it is a STD3
conforming hostname)
flags respectively to be used in the IDNA handling.

RETURN VALUE

getaddrinfo()
returns 0 if it succeeds, or one of the following nonzero error codes:

EAI_ADDRFAMILY

The specified network host does not have any network addresses in the
requested address family.

EAI_AGAIN

The name server returned a temporary failure indication.
Try again later.

EAI_BADFLAGS

hints.ai_flags
contains invalid flags; or,
hints.ai_flags
included
AI_CANONNAME
and
name
was NULL.

EAI_FAIL

The name server returned a permanent failure indication.

EAI_FAMILY

The requested address family is not supported.

EAI_MEMORY

Out of memory.

EAI_NODATA

The specified network host exists, but does not have any
network addresses defined.

EAI_NONAME

The
node
or
service
is not known; or both
node
and
service
are NULL; or
AI_NUMERICSERV
was specified in
hints.ai_flags
and
service
was not a numeric port-number string.

EAI_SERVICE

The requested service is not available for the requested socket type.
It may be available through another socket type.
For example, this error could occur if
service
was "shell" (a service available only on stream sockets), and either
hints.ai_protocol
was
IPPROTO_UDP,
or
hints.ai_socktype
was
SOCK_DGRAM;
or the error could occur if
service
was not NULL, and
hints.ai_socktype
was
SOCK_RAW
(a socket type that does not support the concept of services).

EAI_SOCKTYPE

The requested socket type is not supported.
This could occur, for example, if
hints.ai_socktype
and
hints.ai_protocol
are inconsistent (e.g.,
SOCK_DGRAM
and
IPPROTO_TCP,
respectively).

EAI_SYSTEM

Other system error, check
errno
for details.

The
gai_strerror()
function translates these error codes to a human readable string,
suitable for error reporting.

FILES

/etc/gai.conf

CONFORMING TO

POSIX.1-2001.
The
getaddrinfo()
function is documented in RFC 2553.

NOTES

getaddrinfo()
supports the
address%scope-id
notation for specifying the IPv6 scope-ID.

AI_ADDRCONFIG,
AI_ALL,
and
AI_V4MAPPED
are available since glibc 2.3.3.
AI_NUMERICSERV
is available since glibc 2.3.4.

According to POSIX.1-2001, specifying
hints
as NULL should cause
ai_flags
to be assumed as 0.
The GNU C library instead assumes a value of
(AI_V4MAPPED | AI_ADDRCONFIG)
for this case,
since this value is considered an improvement on the specification.

EXAMPLE

The following programs demonstrate the use of
getaddrinfo(),
gai_strerror(),
freeaddrinfo(),
and
getnameinfo(3).
The programs are an echo server and client for UDP datagrams.

SEE ALSO

COLOPHON

This page is part of release 3.81 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
http://www.kernel.org/doc/man-pages/.